<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        var arr=[1,2,3];
        var items=[];
        items["a"]='html';
        items["b"]='css';
        console.log(items)   //[a:"html",b:"css"]
        var obj={name:"wang",age:18}
        for(let key in obj){
            console.log(key)   //name,age
        }
        
    </script>
</body>
</html>